The present invention relates in general to electronic devices and in particular to a process for charging an electrochromic layered package (mirror) with hydrogen and for applying a backing to an electrochromic layered package (mirror). A layered package of this type comprises a transparent substrate plate disposed on the front, at least two electrodes, where, of these two electrodes, the first after this substrate plate (first electrode) is a transparent electrode, at least one electrochromic layer, a hydrogen-storing layer, a hydrogen ion-conducting layer, and a backing which seals the layered package and immediately follows the second of the two electrodes.
Electrochromic materials are materials which change their optical constants (n and k) when an electrical field is applied, retain this state when the field is switched off, and return to the initial state when the polarity is reversed, the electrochromic material being involved in a redox process.
Typical examples of electrochromic materials are WO.sub.3 and MoO.sub.3, which are colorless and transparent when applied in a thin layer to a glass substrate. However, if a voltage of suitable strength is applied to a layer of this type, suitable ions, for example protons, migrate from one side and electrons migrate from the other side into this layer and form blue tungsten bronze or molybdenum bronze, H.sub.x WO.sub.3 or H.sub.x MoO.sub.3 respectively. The intensity of the coloration is determined by the amount of charge flowing into the layer.
Electrochromic layered packages having optical properties which can be changed at will, in particular having a controllable light absorption, are of considerable interest for a wide variety of uses, e.g., for displays, for transparent optical instruments--spectacles and light valves--and reflective systems--mirrors and reflective displays.
Possible constructions of electrochromic layered packages by means of different layer arrangements are described, for example, in Schott Information 1983, No. 1, p. 11, German Pat. No. 3,008,768, Chemistry in Britain, 21 (1985), 643 or Dechema-Monographien, Volume 102--VCH Verlagssgesellschaft 1986, p. 483.
Electrochromic mirrors as described in German Pat. No. 3,008,768 are constructed exclusively from solid layers, which yields certain advantages over electrochromic mirrors with liquid electrolytes, as described, for example, in U.S. Pat. No. 3,844,636 (e.g. lower thickness of the overall system, no leakage of the acid used as electrolyte on damage and fracture of the system). There are various possible arrangements of the individual layers for the construction of an electrochromic mirror. The layer sequence below (in the viewing direction) is intended merely to represent an example of a possible construction:
glass substrate PA1 transparent electrode PA1 electrochromic electrode PA1 solid hydrogen ion-conducting layer PA1 hydrogen ion-permeable reflector PA1 solid hydrogen ion-conducting layer PA1 hydrogen ion-storing layer PA1 catalytic layer which is simultaneously an electrode PA1 adhesive PA1 sealing plate.
If the reflectivity of the mirror is to be reduced, the absorption of the electrochromic layer is increased by intensifying the color: the transparent electrode is connected as the cathode and the electrode located behind the ion-storing layer is connected as the anode. Protons move frontally (i.e., in the direction of the electrochromic electrode) from the ion-storing layer through layers which are permeable to ions but not electrons, into the electrochromic layer, and electrons pass from the voltage source via the transparent electrode directly into the electrochromic layer. A redox reaction then takes place in the electrochromic material, for example WO.sub.3, with formation of blue tungsten bronze H.sub.x WO.sub.3 : ##STR1## x is known as the depth of reaction and the degree of the light absorption of the electrochromic layer is dependent thereon.
Since the electrochromic reaction is reversible, the reaction can be reversed by reversing the polarity of the electrical field applied to darken the electrochromic layer, and the electrochromic layer is thus lightened again. The electrode disposed immediately in front of the backing (in the example the adhesive and sealing plate) is connected as the cathode, which causes the protons to be transported back into the proton-storing layer: ##STR2##
It is possible here for protons to reach the cathode discharged, where they are.
A rear wall of glass or of bonded or foamed plastic is bonded to the rear in order to seal the system.
Before a layered system of this type is ready for use and can be operated, it must be charged with hydrogen. Hitherto, this charging has been carried out by exposing an appropriate layered package to a hydrogen gas atmosphere while short-circuiting the first and second electrodes; during this operation, hydrogen diffuses into the layered package in an equilibrium reaction via the second electrode, which necessarily comprises a catalytic metal for cleaving the H.sub.2 molecules into H atoms. Another possibility is to connect the first electrode as the negative electrode compared with the second electrode and to allow hydrogen ions to enter the electrochromic layered package via the catalytic, second electrode on application of a voltage. After charging with hydrogen ions, excess hydrogen on the rear of the second electrode must be burnt. Each of the two methods mentioned requires a catalytic second electrode for cleaving the H.sub.2 molecules; in addition, the process for charging from an H.sub.2 atmosphere is very complex since the layered packages must be handled in "glove boxes". After charging with hydrogen, the fact that the sealing of the finished layered package is significantly less than complete has been a problem.
Conventional electrochromic layered packages of the type described here are sealed by applying a backing after charging with hydrogen; such a backing is supposed both to "seal" a layered package of this type and also to protect the second electrode against external mechanical forces or chemical attack. A backing in the form of a bonded plate made of glass, metal or plastic or foamed plastic used hitherto was effectively only to a limited extent.
Only recently, a conventional electrochromic layered package has successfully been hermetically sealed by electrolytic application of a metal layer onto the second electrode, as described in the above cross-referenced application.